docs(worker): document SIENTIA_RUNTIME_READY, log the skipped install, test install failure (SIENTIAPDE-2214)

From the code-review-python pass on #34:
- README, CLAUDE.md and the module docstring: the worker re-executes
  itself after install_runtime, and SIENTIA_RUNTIME_READY is internal;
  setting it in .env/the chart would skip the install silently.
- "Installing runtime" is now logged only on the pass that installs; the
  re-executed pass logs that the install was skipped.
- The ponytail comment notes that the install success metrics are lost
  to the exec.
- New test: a failed install propagates without setting the flag or
  calling os.execv (locks the install -> flag -> exec order).

README.md

@@ -988,6 +988,7 @@ Values are read in `model_manager/utils/connectors_config.py` and `model_manager
 | `TEMPORAL_NAMESPACE` | Temporal namespace | `model-manager` | No |
 | `TEMPORAL_USE_TLS` | Use TLS for Temporal gRPC (`true`/`false`). Set `true` when the endpoint serves TLS or you get HTTP redirects (for example 308) to HTTPS | `false` | No |
 | `RUNTIME` | Suffix for worker task queues (`train_model-<runtime>-queue`, `cleanup_files-<runtime>-queue`) | `single` (via `_get_runtime`) | No |
+| `SIENTIA_RUNTIME_READY` | Internal: set by the worker itself before re-executing after `install_runtime` (SIENTIAPDE-2214). **Never set it in `.env` or the chart**: it skips the runtime install | unset | No |
 | `TRAIN_TASK_QUEUE` | Used by **clients** (for example `scripts/run_training_test.py`), not by the worker process | unset | No |
 | `CLEANUP_TASK_QUEUE` | Used by **clients** (for example `scripts/run_cleanup_test.py`), not by the worker | unset | No |
 | `POSTGRES_HOST` | PostgreSQL hostname | `localhost` | Yes |